And if you don't know what a xenophyophore is, then shame on you! Xenophyophores are sessile deep-sea protists that often reach comparatively gigantic sizes. One species, Stannophyllum venosum (which looks something like half a plate sitting on its edge), can be nearly a foot across (Tendal, 1972). Images from deep-sea submersibles have shown that xenos can be spectacularly abundant, carpeting the ocean floor in some places.

Xenophyophores are sometimes referred to as the largest unicellular organisms, but as I've mentioned before, that's arguably not entirely appropriate. Rather, xenophyophores have a coenocytic or hyphal organisation, with numerous nuclei scattered throughout long branching cytoplasmic tubes. The name "xenophyophore" means "bearer of foreign objects", and refers to the external test of the organism, which it constructs by cementing together objects it collects from the sediment around it - sand grains , for instance, or shells of other organisms - using a polysaccharide cement. The xenos also sequester their faecal pellets, which they may also integrate into their skeleton. Different species of xeno can be distinguished by the nature and arrangement of the foreign particles in the test (they are often quite picky about what they use), and the proportion of foreign particles to cement or faecal pellets. The species Cerelasma massa, for instance, differs from other xenos in using no foreign particles whatsoever, but only cement and its own faecal pellets - hence also being a contender for the title of most disgusting organism in existence. Species that use a high proportion of foreign particles in their construction are generally quite rigid, while those using more cement are softer.

The new xenophyophore species Shinkaiya lindsayi. In the long section, the central white strands are the xenophyophore's cytoplasm, while the black strands are strings of faecal pellets. Figure from Lecroq et al. (2009).

Yesterday saw the publication of a new xenophyophore species, Shinkaiya lindsayi (the genus is named, offhand, after the submersible that was used to collect the type specimen) by Lecroq et al. (2009). One of the most significant features of the new paper is that it includes a molecular phylogenetic analysis of the new species. Xenophyophores have been sequenced on one occasion before, by Pawlowski et al. (2003), who placed the species Syringammina corbicula among basal Foraminifera. It is good to see that Lecroq et al. place Shinkaiya as the sister to Syringammina, and the two together fit in the same position among forams originally found by Pawlowski et al.. The closest relative to xenophyophores identified is a foram called Rhizammina, which is also sessile, constructs a test of foreign matter, and even sequesters faecal pellets in a similar manner to xenophyophores.

This is a noteworthy achievement - these are not easy organisms to sequence. Not only is there a shortage of accessible material, but xenos and forams both tend to have large numbers of bacteria and other micro-organisms living around them, just aching to contaminate DNA samples (the very first molecular phylogenetic analysis of a foram, for instance, suggested a close relationship between forams and dinoflagelates, only to have it later shown that the sequence analysed belonged not to the foram but to parasitic micro-organisms living in the foram*). So the fact that Lecroq et al.'s results are not only well-supported, but make a lot of intuitive sense morphologically, makes this a very nice study indeed.

*Still, if the recently suggested SAR clade is correct, there is a certain irony to this - forams may be somewhat related to dinoflagellates after all.

Haeckel's (1889) illustrations of species of Stannophyllum, including the gigantic S. venosum at the bottom. Stannophyllum belongs to a family of xenophyophores called stannomids that differ from other xenos such as Shinkaiya (called psammetids) in having reinforcing threads (probably made of mucopolysaccharides) running through their tests. Stannomids are more plate-like, upright and flexible than the lumpy psammetids. Image via here (Haeckel referred to xenophyophores as "Deep-Sea Keratosa", as he was under the impression that they were sponges).

REFERENCES

Haeckel E. 1889. Report on the Deep-Sea Keratosa. Report on the Scientific Results of the Voyage of H. M. S. Challenger during the years 1873–76. Zoology 32 (part 82): 1–92.

6 comments:

Phaeodarians also apparently construct shells of fecal matter... although it's not really all that disgusting -- I guess it's fundamentally no different from secreting scales or something, just less discriminatory in the material used...

Would love to compare their cell biol with that of aseptate fungi, vaucheria, myxomycetes and other coenocytic organisms...

Rhizarians are awesome! I know so little about them though =( they're pretty good at escaping attention (and culture attempts, it is rumoured...) Leander lab works on some Cercozoa... they have really pretty pictures! *drools*

As far as I know, there's no real difference between a coenocyte and a syncytium. I think "coenocytic" tends to get used with organisms like fungi or Vaucheria that have a filamentous structure, while "syncytial" is used for more centralised acellular organisms such as ciliates, but I don't know if that's a formal distinction.

This sort of thing happens a lot in biology, and it's mostly an artefact of history - different people working with different groups of organisms may come up with different terms for much the same thing, and then each line of workers tends to follow it's own predecessors.

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The Author

I'm an entomologist and taxonomist, currently based in Perth, Western Australia. If you'd like to comment (or offer work), I can be e-mailed at gerarus at westnet.com.au.Subscribe to Catalogue of Organisms